1. Field of the Invention
The present invention relates to a reactor that is used as an inductance element of an inverter circuit, a converter circuit, and the like.
2. Description of Related Art
Use of a laminated iron core in the magnetic core of a reactor used in a high frequency wave band in an inverter circuit, a converter circuit, and the like can reduce the size of the reactor owing to a high magnetic flux density as compared with the use of other material magnetic cores.
In this type of laminated iron core, eddy current generally increases in the case of a thick laminating material, a small specific resistance of the laminating material, and at high applied frequency waves, thus causing large iron loss. Therefore, the laminating material uses a soft magnetic sheet such as silicon steel sheet having a small thickness and a large specific resistance.
Reactors used in a high frequency wave band conventionally and widely adopt the structure that is illustrated in FIG. 1A (plan view). Coils 2a and 2b are wound around a laminated iron core 11 that comprises iron cores 11a and 11b which have a square cross section and which are laminated with soft magnetic sheets 110. Normally, each of the coils 2a and 2b comprises a rectangular conductor wire 20 wound in an upright orientation. Since the rectangular conductor wire 20 has high rigidity, it cannot be wound in a square pattern along the surface of the laminated iron,core 11. Thus, the rectangular conductor wire 20 is wound in a circular pattern as shown in FIG. 1B (which is a sectional view along line Ixe2x80x94I in FIG. 1A).
In recent years, it has been desired to utilize higher frequency wave bands to achieve size reduction and increased power source efficiency. Accordingly, the desire for low noise and low loss (i.e., low iron loss and low conductor loss) in reactors has been increased. The reactors of conventional design, however, face the problems described below:
(1) Since adjacent portions of the rectangular conductor wire 20 touch each other, the series capacitance between adjacent portions of the conductor wire 20 is large. Accordingly, switching noise caused from the leakage of high frequency waves via parasitic capacitor becomes significant. As a result, external noise countermeasures are required.
(2) Since the coils 2a and 2b are located in a proximity arrangement to minimize the reactor size, the parallel capacitance between the coils 2a and 2b becomes large. Consequently, resonance current occurs in the coils 2a and 2b when a square wave current is OFF, which resonance current worsens the switching noise characteristic.
(3) Since the coils 2a and 2b are located in a proximity arrangement, an insulation material to assure insulation dielectric strength is required.
(4) Since adjacent portions of the rectangular conductor wire 20 touch each other, the proximity effect increases the alternating effective efficiency so as to generate calorific loss on the coils 2a and 2b. 
(5) Since adjacent portions of the rectangular conductor wire 20 touch each other, the contact faces between the coils 2a, 2b and air are limited to the side faces of the rectangular conductor wire 20 (outer peripheral surface of the coil). In addition, since the coils 2a and 2b are located in a proximity arrangement, effective heat dissipation cannot be achieved. As a result, the size and the weight of the reactor has to be increased so as to increase the heat releasing surface area, and further an insulator is required, which results in increased material costs.
(6) Since the coils 2a and 2b are separately wound in a circular pattern along the laminated iron core portions 11a and 11b having a square cross section, a space exists between the coils 2a, 2b and the laminated iron core portions 11a and 11b, which increases the iron loss. A laminated iron core 11 having a square cross section, moreover, has a longer winding length of coil than that of a laminated iron core having a circular cross section of the same cross sectional area. As a result, the conductor resistance (which is equal to direct current resistance+skin effect+proximity effect) in the former type of laminated iron core increases, thus increasing the conductor loss of the reactor. And if the inner diameter of the coils 2a and 2b is the same, the cross sectional area of a laminated iron core 11 having a square cross section is smaller by about 36% than that of a laminated iron core having a circular cross section. As a result, the magnetic flux density in the former type of laminated iron core increases, thus increasing the iron loss of the reactor. Furthermore, a laminated iron core 11 having a square cross section results in a large space between the coils 2a, 2b and the laminated iron core portions 11a and 11b, as described above, so that the prevention of vibration and noise is difficult.
(7) Since the coils 2a and 2b are formed by winding respective rectangular conductor wires in a straight cylindrical shape, leaked magnetic flux from edges of the coils 2a and 2b is significant.
(8) A thin insulating film is formed on the surface of the laminating material of the laminated iron core 11 to prevent short circuiting. Since, however, same size soft magnetic sheets 110 are laminated, a burr generated on the cut sections of a soft magnetic sheet 110 contacts a sagging portion of an adjacent soft magnetic sheet 110, which destroys the insulating film to induce a micro-short circuit. As a result, the iron loss is significantly increased particularly in a high frequency wave band. And because burr formation and sagging are inevitably generated during shearing, complete prevention of a micro-short circuit is difficult.
(9) Clamping members to fix the plurality of iron cores 11a and 11b to each other are required, which results in a large number of assembly working hours. And if the clamping members are made of a conductive metal, an insulation treatment against the coils 2a and 2b is further required.
(10) A specified direct current convolutional characteristic is obtained by inserting a specified gap material 13 between respective iron cores 11a and 11b. Therefore, gap clamping members are required.
(11) Since the plurality of iron cores 11a and 11b are fixed to each other, individual iron cores 11a and 11b are subjected to electromagnetic vibration of high frequency waves, which likely induces the generation of vibration noise or resonance noise.
(12) Since the iron cores 11a and 11b are fabricated by laminating soft magnetic sheets 110, the number of work hours for shearing, adhering, and the like significantly increases.
(13) To discard the reactor, the treatment cost is significant because the kinds of materials for disassembling and separating are many.
An object of the present invention is to provide a reactor which generates low noise and low loss without inducing the above-described problems.
The object is achieved by providing a low noise and low loss reactor which comprises a wound and laminated iron core formed by winding a soft magnetic thin strip into a circular ring shape or elliptical ring shape, and a coil wound around almost an entire outer periphery of the ring of wound and laminated iron core, wherein a cross sectional shape of the wound and laminated iron core vertical to a peripheral direction of the ring is any one of: (i) a circular shape, (ii) an elliptical shape, (iii) a substantially regular polygon of at least 6 sides, (iv) a shape encircled by a pair of point-symmetrically positioned circular arcs or elliptical arcs with a nearly straight line connecting respective edges of the pair of circular arcs or elliptical arcs on both sides of the pair of circular arcs or elliptical arcs, and (v) a shape of a substantially regular polygon of at least 4 sides whose apexes comprise a circular arc or an elliptical arc.